This application is directed generally to the metal detection arts and more particularly to a novel and improved monitoring circuit for a metal detector system.
While the invention may find utility in other applications, the disclosure will be facilitated by addressing the problem of detecting the presence of tramp metal in the crop mat ingested by a forage harvesting machine.
Conventional forage harvesting machines generally utilize a pair of rotating feed rolls at an intake end thereof for delivering crop material into the harvesting machine for further processing. In operation, it is desirable to eliminate any metal objects or material which may be present in the incoming crop mat. This is done to avoid possible deleterious effects of such metal objects or material to livestock ingesting the forage, as well as possible damage to the harvesting machine.
Conventionally, suitable electromagnetic coil-type sensors have been utilized to detect the presence of such "tramp metal" in the incoming crop mat at the feed rolls. Cooperatively, a clutch or braking mechanism is responsive to detection of metal by this sensor and associated monitoring circuitry for quickly stopping the rotation of the intake or feed rolls. Accordingly, it has heretofore been the practice for the operator to then rotate the feed rolls in the reverse direction and inspect the crop mat so as to remove any metal objects or material located therein.
Suitable monitoring and control circuitry is provided between the metal sensor and a clutch or other mechanism to stop the feed rolls. This circuitry also activates some suitable alarm to alert the operator. Conventionally, this circuitry includes a suitable bandpass filter and a detection threshold-type circuit. These circuits are provided to reject spurious or noise signals or the like so as to assure operation of the alarm and of the clutch or braking mechanism only in response to actual detection of metal by the sensor. Such spurious or noise signals may be generated by imperfections in the feed rolls themselves which are generally fabricated of stainless steel, or due to other rotating metal machine parts.
It is known to eliminate much of the machine-generated spurious signals of the foregoing types by the use of a simple bandpass filter. Additionally, the threshold circuit eliminates relative low amplitude noise signals, thus substantially assuring triggering of the alarm and clutch or braking mechanisms in response only to the presence of tramp metal as detected by the sensor.
While the foregoing circuit arrangement has found widespread acceptance, there is room yet for further improvement. For example, the conventional circuitry has comprised a fixed bandpass filter and a fixed detection threshold.
However, performance trade-offs are inherent in such a system, due primarily to the variable angular velocities or rotational speeds at which the feed rolls may be operated. For example, at low feed roll speeds, both the spurious signals and the signals generated by the metal sensor are relatively low in frequency and amplitude. Hence, a relatively low center frequency of the pass band and low threshold of detection are suitable under such conditions.
However, the amplitude and frequency of both the spurious signals and the sensor signals increase with increasing speed roll velocities. Hence, at higher feed roll velocities a relatively higher pass band center frequency and higher threshold amplitude are desirable. Accordingly, a fixed bandpass filter and fixed threshold necessarily result in some performance compromise over the range of rotational speeds which may be expected to be encountered during typical operation of the feed rolls.
A simple widening of the bandpass filter helps to detect a broader range of frequencies of sensor signals, however, at higher frequencies, higher amplitudes are also encountered, whereby a higher detection threshold is necessary. However, with a relatively high detection threshold, there is substantial danger of failing to respond to relatively low amplitude sensor signals which occur at lower velocities, and hence failing to respond to the intake of metal.
Additionally, it is often desirable to again detect and verify the presence of the metal object or material during reverse rotation of the feed rolls. As mentioned above, the feed rolls are rotated in the reverse direction upon detection of incoming metal to permit removal of the metallic material from the crop mat. However, this reverse rotation is generally at a relatively low speed or angular velocity, and usually at a lower rotational speed than the normal forward rotational speed of the feed rolls. Accordingly, the same problems noted above with respect to reliable detection of metal at varying feed roll speeds is also encountered during reverse rotation.